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Case Study: Amplifier Loop Gain Testing in B² Spice

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Measuring Loop Gain
Most amplifier use feedback to extend bandwidth, lower distortion and noise and output impedance. The feedback is fueled by having more open-loop gain than closed-loop gain. The interesting problem that comes up in SPICE simulations of feedback-based amplifiers is how to measure the open-loop gain. Interesting because it is difficult to test an amplifier with an open loop, whether in reality or in simulation. Opening the loop will throw off the DC bias points, which rely on the feedback loop to stabilize, without the feedback the output latches up to one of the power supply rails. So, the secret is not to open the feedback loop, while measuring the open loop gain.

First we add an AC voltage source in series with the output and the feedback resistor. Its DC value is set to 0 volts and the AC voltage is set to some nominal value, such as 10mV.

Then we measure the gain experienced at the output relative to the inverting input, which allows the DC points to stay in place. In other words, the output voltage is divided by the signal at the inverting input and this result equals the open-loop gain.

More information on this technique is outlined in an article by Dr. R. D. Middlebrook in International Journal of Electronics, volume 38, number 4, 1975.